Oh man, I've had a few hair mistakes! 

The first was when I was 12 years old - my mother convinced me to get a perm. So my hair lady (same one I've had for almost 20 years now) gave me a home perm setting.    Instead of sexy spirals, the strands had more of a Z-shape as if I'd had a terrible crimping accident or stuck my finger in a light socket! All the gel & water in the world wouldn't tame it, so I rocked the shock treatment look for a few months before discovering that I could blowfry my hair straight. So I just did that until the electroshock perm disappeared.

The second was when I was 20. During the week between my wedding and honeymoon, I decided to dye my natural light brown tresses black. As a goth, I'd of course done this a few times before with a semi-permanent and it always washed out in time. When I went to purchase my usual semi-permanent black, however, it was not in stock. So I bought a demi-permanent instead.
    Because of the huge difference between my natural color and the artificial color, the dye took permanently and never washed out. When my roots began to show, I used another demi-permanent but in an indigo-black this time. This covered my roots and gave a beautiful blue-ish "moonlit" sheen to my already black hair. But it wasn't long before I wanted my natural haircolor back.
    Rather than having the color chemically stripped (which I couldn't afford to do), I opted to cut my waist length hair to just above shoulder length. My roots weren't very long at the time, so I rocked the 2-tone look for about a year: light on top, dark on the bottom. Yikes. 
    When my roots had grown to 6", I finally cut all of the dyed hair off. And I'm happy to report that I've not used haircolor since!!